Particles at a distance

The aspirated powder is dispersed immediately and with a high shearing effect

Powder handling generates dust. But that is not the only problem. In the vast majority of cases, powders are agglomerated - and the finer a powder is, the more it tends to agglomerate. The causes are manifold.

In addition to physical and electrostatic forces (Van der Waals, Coulomb, sintering bridges), there are numerous other causes for the formation of agglomerates in everyday production. For example, temperature increases above the glass transition temperature during transportation cause particles to fuse together (caking). Cool storage can lead to the temperature falling below the dew point, to condensation within the powder and thus to liquid bridges between the particles.

The problem of agglomerates
In order to achieve the best possible dispersion result, existing agglomerates must be broken down immediately during powder application and the formation of new agglomerates during powder application in liquids must be avoided from the outset. Otherwise, these agglomerates have to be subsequently broken down by prolonged stirring and time-consuming post-dispersing - often with negative consequences for product quality: in yoghurt, for example, the texture is destroyed and additional proteins and stabilizers are required; in shampoos, the viscosity drops and additional thickeners have to be used. Polymers are destroyed and resins or binders are overheated. In addition, post-dispersion costs time and energy and unnecessarily blocks the process containers.

The powder particles must therefore be separated before they come into contact with the liquid and each particle must be completely wetted individually. The particle surface to be wetted during powder application is huge. It is between 1 and 1,000 m²/g powder. A 25 kg bag of powder can therefore have a particle surface area to be wetted of between 25,000 m² and 25 km². In addition to this outer surface, porous particles such as silica gels also have an inner surface - and this must also be completely wetted.

Powders also contain a lot of air. Even heavy powders such as titanium dioxide contain more than 75% air by volume. With light powders, the proportion can be over 95%. This air must be completely replaced by liquid and separated. It must not be dispersed with the powder, as this leads to microfoam.

Lumps, crusts, foam
Conventional methods of adding powder to liquids using agitators, injectors or inline mixers mainly produce undesirable agglomerates. The powder particles do not come into contact with the liquid individually, but as a compact bulk. The liquid surface available to the powder for wetting is orders of magnitude smaller than the powder surface to be wetted. This results in stable, partially wetted agglomerates that are difficult to break down.

When powder is added to an open container from above, these problems become particularly obvious: partially wetted lumps form on the surface of the liquid. In the worst case they float on the surface, in the best case they sink. Dust above the liquid leads to adhesions on moist surfaces as well as to powder crusts and soiling on the container wall, container lid, agitator shaft and all installations in the container. The crusts later crumble into the product and reduce the quality. If an extraction system is used to avoid dust, an uncontrolled amount of the powder is lost in the filters. In addition, when the powder is stirred into the open container, it forms clumps that introduce additional air into the liquid.

Suction conveyors, which transport the powder into the container with little dust, still generate a lot of dust above the liquid. The wetting problems in the container are therefore not avoided, but rather intensified. Agglomerates also inevitably occur in vacuum process containers because the particles are not separated before contact with the liquid and the liquid surface is far too small for complete wetting. At the same time, tumbling occurs and there is a risk that some of the powder will be sucked off by the vacuum pump without being wetted and thus lost. Even in the case of injectors with an upstream or downstream pump, the liquid surface area is far from sufficient. A dispersing machine is often installed downstream of such a system in order to break down the agglomerates introduced. However, the air contained in the powder is dispersed particularly finely, which significantly impairs the dispersing effect and produces stable microfoam.

Vacuum expansion: completely wetted in microseconds
The weaknesses of these conventional powder wetting processes are avoided with the Ystral Conti-TDS inline dispersing machine developed by Ystral. The machine completely deagglomerates and wets powder and replaces air with liquid within microseconds.

The inline dispersing machine is operated on one or more process vessels. It conveys the liquid in a circuit and sucks the powder into the liquid from a bag, hopper, big bag, silo or container. The powder particles are separated according to the principle of vacuum expansion: The air contained in the powder is expanded many times over, greatly increasing the distances between the particles. This allows the particles to be separated and fluidized without additional air.

Powder and liquid only come into contact with each other in the wetting chamber - under vacuum and strong turbulence. In the dispersing zone, the powder particles have the greatest possible distance from each other and can therefore be completely wetted and dispersed individually. The machine generates a liquid surface area of around ½ million m² per minute. This is more than is required for complete wetting.

The air previously contained in the powder is separated from the significantly heavier dispersion by the centrifugal effect of the fast-running rotor and coalesces into large air bubbles. These are then conveyed together with the liquid flow to the process container, where they can easily escape.

The vacuum expansion method offers great economic advantages: In paint production, for example, costs can be reduced by more than 90% in this way, while resins can be dissolved in 1/50th of the time - and with significantly improved product quality.